The invention concerns an automatic transmission.
The automatically controlled transmission has a continuously variable ratio step with a primary variator pulley and a secondary variator pulley, which are drivingly interconnected by a belt-type organ in the form of a chain or belt. Such so-called CVT transmissions, continuously variable transmissions, are known already, e.g. from ATZ Automobiltechnische Zeitschrift 96 (1994), pp. 578 to 587. Aside from the variator, a disc clutch in the form of a hydrodynamic torque converter with lock-up clutch is provided on the input side of the transmission. A pump wheel of the hydrodynamic torque converter is driven by an internal combustion piston engine as the prime mover and is non-rotatably connected with a power part of a hydraulic pump, such as a radial piston pump.
Situated between the variator and the hydrodynamic torque converter is a reversing set in planetary design which has one clutch for the forward drive and one brake for the reverse drive. For adapting the variator thrust to a convenient total ratio of the transmission, an intermediate step is rear-mounted on the variator. The intermediate step can also serve as adaptation of direction of rotation and also takes care of an output coaxial with the input shaft.
Each one of the primary and secondary variator pulleys has a cone pulley fixedly connected, respectively, with the primary shaft and secondary shaft and an axially displaceable cone rotatable therewith. The displaceable cones have a piston chamber for the hydraulic axial adjustment. To change the reduction ratio, an electronic regulating device, of the transmission, controls the piston chambers of the movable cones, the pressure medium reaching the rotating piston chambers via ducts in the shafts.
Depending on the rotational speed, the pressure medium is subject to different centrifugal forces which override the control and must be accordingly taken into account. In addition, the pressure medium in the rotating piston chambers increases the masses of the rotating parts and therewith the drag torque which acts negatively, the required contact pressures and thus upon the efficiency of the transmission when adjusting speed. The torque converter exerts a similarly negative influence in relation to the drag torque of the transmission even when the lock-up clutch is closed in order to keep the hydrodynamic losses in the converter low in some operating ranges. CVT transmissions are basically known for small cars, which have a simple starting element in the form of a magnetic powder or multi-disc wet clutch, instead of the hydrodynamic torque converter.
The problem, on which the invention is based, is to reduce the drag torque of the rotating parts and the flow rate need of the hydraulic actuators, the same as to improve the operating behavior.
According to the invention, the starting element is a dry-disc clutch whose disc is non-rotatably connected with the primary shaft and interacts in a clutch housing with a non-rotatably, but axially movably disposed pressure plate. A non-rotating actuator conveniently situated on a clutch bell housing actuates via an axial bearing the pressure plate, which presses the disc against a friction surface in the clutch housing. The clutch housing itself, which can form part of an inertia mass of a prime mover, is non-rotatably connected with the input shaft of the prime mover, such as a crankshaft or a flywheel connected therewith.
In open state, the dry disc clutch clearly separates the prime mover from the transmission without the tow torques being transmitted to the primary shaft by the fluid friction. Further creep of the vehicle is prevented when parked. The rotating masses of the disc clutch, connected with the transmission, are very small. They consist only of the disc connected with the primary shaft while the actuator is situated in the clutch bell housing and does not rotate.
The actuator conveniently consists of a hydraulic piston which, designed as an annular piston, in an annular cylinder of the clutch bell housing and is loaded with hydraulic oil by a transmission oil pump, the control being effected via an electronic pressure-control unit of a control unit of the transmission. The hydraulic piston closes the clutch while a recoil spring opens the disc clutch. It is thus ensured that, in case of failure of the hydraulic system, the clutch is brought to the safe open state. The power part of the transmission oil pump is non-rotatably connected with the pressure plate. Thus, it is driven at the rotational speed of the prime mover and thereby consumes only a small amount of power at idling speed and in a low rotational speed range, whereby the efficiency of the transmission is improved.
According to a development of the invention, two redundant annular pistons act upon the pressure plate, via the axial bearing, so as to ensure the driveability as long as possible. In normal operation, a first annular piston is controlled by means of the transmission oil pump, while a second control unit controls the second annular piston by means of a pressure supply pump, independent of the transmission oil pump, when a first electronic control unit and/or the hydraulic system of the transmission fails and/or a electronic control unit detects an ABS operation. By ABS operation is understood an operation state in which an anti-lock system reacts.
An independent pressure supply can serve the ABS hydraulic system and the oil supply of an auxiliary steering pump, the second electronic control unit can be integrated in an engine control unit or in an ABS control unit. If one of the electronic control units detects an ABS operation, drive line slip losses are reduced when the anti-lock system is active.
The hydraulic pistons are conveniently designed as annular pistons and consecutively shifted in series. They can be shifted into each other so that one piston serves wholly or partly as a cylinder for the other hydraulic piston. The annular pistons surround the extension of the primary shaft and the drag between the pressure plate and the transmission oil pump. The drag is designed as a hollow shaft and supported on the extension of the primary shaft. Thereby a small construction volume results and, in addition, a good reaction behavior of the clutch is obtained by the short hydraulic connections.
According to a development of the invention, in order further to reduce the rotating masses, cylinder-piston units, which adjust the movable cones of the variator, are situated fixed to the housing. Thereby is obtained that the hydraulic medium is not exposed to any centrifugal forces which must be taken into account in the control. The forces, which are exerted upon the variator pulleys by the cylinder-piston units, support themselves in the transmission housing via the primary shaft or the secondary shaft and the appertaining fixed bearing. Between the cylinder-piston units and the adjustable cones, axial bearings are provided which reduce the friction. In this case, relatively short hydraulic ducts fixed to the housing can also be designed with a large cross-section whereby the response behavior of the variator is improved. Finally, it is possible to omit the otherwise usual dynamic pressure-compensating chamber and expensive seals between parts having different rotational speeds.
According to one other development of the invention, a reversing set in planetary design is situated on the secondary shaft. In addition to a sun gear and an internal geared wheel, it has a double planet so that the reduction ratio of the reversing set can be equally designed in both directions. The total ratio of the transmission and the adaptation, specific to the vehicle of the direction of rotation during forward drive, can be determined by a rear-mounted intermediate step. The sun gear of the reversing set is non-rotatably connected with the secondary shaft and in forward drive range can be coupled via a clutch with a planet carrier. For reverse drive, the clutch is opened and the internal geared wheel slowed down by a brake. For the output, an output gear wheel is non-rotatably connected with the planet carrier.
To be able to transmit sufficient torque in the forward drive, it is convenient that the internal disc carrier of the clutch designed as disc clutch be connected with the planet carrier and the external disc carrier with the secondary shaft and the sun gear. For the friction surfaces thus results a larger, more effective diameter based on the secondary shaft.
The brake is conveniently designed centered in position as one-disc brake, wherein the brake disc fastened to the internal geared wheel is situated in a brake caliper between two pistons which are controlled via a brake valve. Together with the small rotating mass of the brake disc, the advantage results that the two brake pistons in the double-piston arrangement, such as has proved satisfactory for vehicle service brakes, quickly release the brake disc during pressure break down by slightly recoiling. Thereby are clearly reduced drag losses in the forward drive operation in which the reversing gear set rotates as block.
The annular piston for actuating the starting clutch of the transmission hydraulic system is conveniently controlled by an electronic pressure-control unit via a pressure-control valve. The characteristic of the pressure-control unit is laid out so that the appertaining cylinder of the first annular piston is ventilated when the first electronic control unit of the transmission passes to emergency operation. The disc clutch opens and thus changes to the safety state.
When the first electronic control unit, which is capable of regulating the transmission, passes to the interference mode bit, the second annular piston is conveniently controlled directly by an electronic pressure-control unit placed in the transmission. At the same time, the second piston is supplied with energy by an independent pressure supply, such as an auxiliary steering pump or an anti-lock system. The control of the second annular piston can be integrated in an electronic engine control or can be effected by a pressure-control valve integrated in an ABS block.
The first electronic control unit of the transmission, the electronic control unit of the anti-lock system and the electronic control unit of the prime mover conveniently communicate, via a CAN bus (controller area network), and thus exchange data relevant to the control. The characteristic of the control of the second annular piston is designed so that, in case of power failure, it ventilates the appertaining cylinder and thus the disc clutch is opened. Thus, in case of failure of the transmission control, the engine control takes over the control logic for the disc clutch. Besides, the ABS control unit can for its part improve during ABS operation, the regulation excellence and the driving stability by its opening during ABS operation, via the second annular piston, the disc clutch and thus clearly reducing the drive line drag torque by uncoupling the prime mover.
The movable cones of the variator can be conveniently controlled, according to the xe2x80x9cmaster-slave principlexe2x80x9d, i.e. the pressure in the cylinder-piston unit for the secondary variator pulley corresponds to the secondary torque to be transmitted, and the pressure of the cylinder-piston unit for the primary variator pulley is determined by a ratio regulator according to the selected cylinder surface ratio. The surfaces of the cylinder are ideally selected so as not to fall below the contact pressure need of the secondary variator pulley, taking into account the smallest possible system pressure for maintaining the pre-control pressure supply of about 6 bar. The theoretical minimum pressure for constant drive in the total load and rotational speed range must likewise be laid out on the value.
The movable cones of the variator can also be controlled according to the xe2x80x9cpartner principlexe2x80x9d, i.e. equal surfaces are used for the cylinder-piston units of the primary side and secondary side of the variator and the ratio is controlled via a square slide.
In both cases, since the cylinder-piston units are stationary in the housing and not exposed to any centrifugal forces, an oil supply for a dynamic pressure-compensation chamber and the latter, itself, can be omitted.
In one other development of the invention, it is proposed that the transmitting capacity of the clutch and brake of the reversing gear set be selected so that the pressure on the secondary variator pulley, including the required safety factor, is directly used as operation pressure for the clutch and brake. Thereby it is possible with an electronic pressure-control unit to make the contact pressure for the cones, the clutch pressure and the brake pressure available.
It is also advantageous that the reversing gear set be shifted only when the disc clutch is open and the shifting process be controlled by pressure ramps on the pressure on the secondary variator pulley. It is possible to implement the shifting process with a good shifting comfort without volume damper for the clutch and/or the brake.
The brake of the reversing gear set is actuated by a precontrolled brake valve via a closed system. In reverse drive, the working piston of the brake valve is connected directly, via a shift valve, with the pressure of the secondary variator pulley by the position of the shift valve and the working volume of the working piston joins the pistons to the brake disc against the force of a spring. In the neutral position of the selector lever, the spring moves the brake piston and the working piston back to the initial position. Thereby the brake disc can freely move causing no drag losses either in the forward drive or in the neutral position.
In this system, since the variator cannot be uncoupled in the sense of a parking adjustment, in order to assist a return to xe2x80x9clowxe2x80x9d, it is further proposed that for pressure control in the clutch or in the brake, a pressure-regulating valve be front-amounted on the shift valve.
Finally, it is advantageous for control of the operation cycle, for regulation of the variator ratio and regulation of the disc clutch to measure by Hall sensors the rotational speed of the prime mover on the clutch housing, the rotational speed of the primary variator pulley on the primary shaft, the rotational speed of the secondary variator pulley on the secondary shaft and the initial rotational speed of the output shaft on a differential.